Generally used as an optical source of a transmitter apparatus of an optical transmission system is a laser diode (hereinafter called laser device). Although the oscillation wavelength of a laser device is determined by the cavity length, since the refractive index of the cavity varies with temperature of the laser device, the cavity length also varies equivalently. To prevent this, an electronic cooling element such as Peltier element has been used conventionally to stabilize the temperature of the laser element.
However, even if the temperature is held constant, since the laser bias current increases with aged deterioration, the oscillation wavelength also varies. Moreover, aged changes, deterioration and troubles of a temperature detector element such as thermistor, temperature control element such as Peltier element and temperature control circuit for driving the temperature control element in response to output of the temperature detector element may disable the control for stabilizing the temperature at a desired value. Due to these facts, the optical output wavelength of the laser element may deviate from the desired value.
Especially, in wavelength-division-multiplex optical transmission, control must be made to stabilize each optical wavelength, and particular consideration is required to prevent any affection to optical signals with other optical wavelengths. Shown below are some conventional apparatuses for monitoring individual optical wavelengths in wavelength-division-multiplex optical transmission systems. FIGS. 16 and 17 are block diagrams of general constructions of conventional apparatuses used to multiplex eight optical wavelengths.
Explanation is made on FIG. 16. Optical transmitters 310-1, 310-2, . . . 310-8 respectively include laser elements for oscillation in different optical wavelengths and modulator elements for modulating the laser optical outputs by input data. Respective optical outputs from the optical transmitters 310-1, 310-2, . . . 310-8 are each divided into two parts by optical dividers 312-1, 312-2, . . . 312-8. One of the two parts is applied to the optical composer 314, and the other is applied to a channel selector 316. The optical composer 314 composes optical signals from the optical dividers 312-1, 312-2, . . . 312-8, namely, wavelength-division-multiplexes these signals, and supplies its output to an optical transmission line (optical fiber). The channel selector 316 selects one of optical signals from the optical dividers 312-1, 312-2, . . . , 312-8, and applies it to a wavemeter 318.
The wavemeter 318 has built-in wavelength reference light, and can measure optical wavelengths with quite a high accuracy by using a Michelson interferometer. However, the Michelson interferometer basically has to continuously move a reflector, such as corner-cube. That is, it includes a movable portion. Apparatuses of this type are acceptable for use in laboratories, but not suitable for use in transmission facilities that must maintain a reliability for a continuous, long period of time.
Another conventional apparatus shown in FIG. 17 is explained. This apparatus measures respective wavelengths from a wavelength-multiplexed optical signal. Optical transmitters 320-1, 320-2, . . . , 320-8 respectively include laser elements for oscillation in different optical wavelengths and modulator elements for modulating the laser optical outputs by input data. Respective optical outputs from the optical transmitters 320-1, 320-2, . . . 320-8 are composed, namely, wavelength-division-multiplexed, by an optical composer 322. An optical divider 324 divides the optical signal output from the optical composer 322 into two parts, and applies one of the parts to a transmission line (optical fiber) and the other to a monitoring apparatus 326 that can collectively measure individual optical wavelengths of multiple channels.
The monitoring apparatus 326 practically includes a wavemeter capable of measuring a plurality of wavelengths collectively based on the same theory as the wavemeter 318 or an optical spectrum analyzer for measuring the wavelength of a signal from a peak value obtained by sweeping a spectroscope, and displays and/or prints out the result of the measurement.
Whichever one of a wavemeter or an optical spectrum analyzer is used as means for confirming whether an optical wavelength is held at a predetermined value or within a predetermined range, it is necessary to mechanically sweep some movable element. Since this method uses a movable element, it is not suitable for use in transmission facilities that are required to be reliable for a continuous, long period of time.
It is therefore an object of the invention to provide an optical wavelength monitoring apparatus solving the above-mentioned problem and capable of easily confirming whether an optical wavelength is held within a predetermined range.
Another object of the invention is to provide an optical wavelength monitoring apparatus less affected by environments.
Another object of the invention is to provide an optical wavelength monitoring apparatus capable of maintaining a high reliability over a long span of time.